How does the image caption feature work?

You can enable the image caption feature with the Show image caption with the product name option in the app preferences (Zoom preferences):

When the option is enabled and your images code (the title or the alt attributes) contains a description of the image, it will be displayed a the bottom of the image:

You can add alt texts to your images from your Shopify admin following the instructions at https://help.shopify.com/manual/products/product-variant-images#add-alt-text-to-product-images